Discrete element modeling of the structural vergence beneath intermediate décollements: Implications for along-strike variations in the Zagros Simply Folded Belt

In thrust wedges with multiple weak layers, individual décollements are generally thought to control only the structural vergence of the thrust systems directly above them. However, along the strike of the Zagros Simply Folded Belt, the structures above a common basal décollement at the Hormuz salt layer exhibit different vergence, including forward-, doubly-, and backward-vergent thrust wedges. Despite the importance of intermediate décollements for determining structural styles in the Zagros Simply Folded Belt, their effects on the structural vergence of the thrust system beneath them remain poorly understood. To address this issue, we conducted systematic experiments using discrete element models with a basal décollement on top of the basement and an intermediate décollement within the overlying strata. By varying the strength, thickness, and depth of the intermediate décollement, we successfully reproduced all types of thrust wedges (i.e., forward-, doubly-, and backward-vergent) in the thrust system beneath it. This suggests that intermediate décollements can determine the vergence of the thrust wedges beneath them. Moreover, our results reveal that specific combinations of intermediate décollement properties and shortening rates are crucial for the development of a backward-vergent thrust wedge in the thrust system beneath the intermediate décollement. This provides an alternative mechanism for the formation of such wedges. Together with geological and geophysical observations, we propose that intermediate décollements are responsible for along-strike vergence variations in the Zagros Simply Folded Belt. Thus, the interactions between thrust systems at various scales should be emphasized.